Dissolved Oxygen (DO)
This type of measurement employs an electrode (comprising a cathode and anode usually inside a hollow cylindrical body) containing a 'filling solution' with the measuring end of the electrode covered with a plastic (usually Teflon or polethylene) film, across which oxygen molecules can diffuse.
Galvanic oxygen electrodes use a platinum cathode and a lead anode.Oxygen is spontaneously reduced at the cathode while the lead anode is slowly converted to lead oxide. The amount of current passed from the anode to the cathode is proportional to the oxygen content of the solution. This is the principle of the:
This electrode has an internal 'shorting resistor' so that the current flow is converted to a voltage signal so that it can be directly connected to any e-corder . This type of electrode is OK for student or routine use is usually avoided for accurate research use.
The 'Clark', (or polarographic, or amperometric) method uses a cathode that is held at a 'polarising voltage' of -0.7 V to -0.8 V which causes the reduction of oxygen, (i.e. it is converted to hydroxide). The anode is commonly made of silver. The current flow from the anode to the cathode is proportional to the oxygen content of the solution. These electrodes require a special meter to provide the polarising voltage and measure the current flow and convert it into a signal that the e-corder can record. Some meters may allow the polarising voltage to be adjusted for optimum use. The
- EP354 dO2 isoPod and e-corder
- EPU354 dO2 isoPod (single channel) or Quad MF isoPod (four channel) with USB connection, using Pod-Vu, LabVIEW, or other software.
can be used with most polarographic electrodes and, because they are electrically isolated, they will not interfere with nearby pH or temperature probes.
Almost all research work is done with Clark-style polarographic electrodes. eDAQ offers:
- ET1120 Micro Oxygen Electrode a miniature Clark oxygen electrode for small volume samples
- ET1117 Polarographic Oxygen Electrode a traditional oxygen electrode.
Other Clark electrodes and meters can be obtained from:
- Microelectrodes Inc , Miniature electrodes. See the MI-730 electrode (or 8-730 or 16-730 'Flow Thru' models).
- ABTech Scientific OS-418 miniature oxygen sensor, OSI-2 meter
- Instech See model 203 dual channel meter. Also have accessories for cell/mitochondrial respiration experiments.
- Rank Brothers Also have accessories for cell/mitochondrial respiration experiments.
- Mettler-Toledo meters and electrodes
- Innovative Instruments
- Phoenix Electrodes
- Thermo Fisher Scientific
- Lazar Laboratories, DO-166NP pO2 probe. Incorporates sufficient circuitry so that it can be connected to a e-corder directly, without the need for a separate meter.
If you buy the meter and electrode from the same manufacturer so make sure that the meter has an analog recorder output so that it will be compatible with the e-corder unit.
Both Clark and galvanic oxygen electrodes suffer the disadvantage that the oxygen is consumed during the measurement process. Thus you should ensure that the electrode you choose will consume oxygen at a rate much less than the reaction that you are trying to monitor. In practice this is usually not a significant problem, but if you are studying cell or mitochondrial respiration in a small, hermetically sealed volume of solution, then you should select a small electrode (the smaller the active surface area of the electrode the smaller the oxygen consumption). Manufacturers supply oxygen consumption specifications for their electrodes. On the other hand the small signals obtained from such electrodes are more subject to inteference so shielding of the experiment is often important.
Because both Clark and galvanic oxygen electrodes depend on the diffusion of oxygen from the sample solution into the electrode, their response time is slow - usually several seconds at best. Thus only slow data acquisition rates (4 /s or slower) are usually required. You can also employ the
e-corder low pass filters, down to 1 Hz, to ensure a noise-free signal. It is also imporant to stir the solution at a constant rate throughout the experiment to ensure that the rate of diffusion also remains constant.
Optic Fibre Systems
Oxygen concentration can also be determined with special fibre optic probes and meters. These systems are more expensive than the use of Clark or galvanic electrodes, but have the advantage of zero oxygen consumption, as well as electrical isolation.
- Oxford Optronix, OxyLab pO2 system
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